1. Field of the Invention
The present invention relates to a method of and an apparatus for storing image data in a color image reading system which has three R, G, B linear image sensors housed in a single semiconductor package, each comprising an array of photoelectric transducer pixels arranged in a main scanning direction, for photoelectrically reading reflected or transmitted light which represents image information carried by a subject.
2. Description of the Related Art
Conventional color image reading systems operate by applying illuminating light to a subject placed on a subject table, guiding light representing image information carried by the subject as reflected or transmitted light to a focusing optical system, supplying the light to tricolor separating prisms, and photoelectrically reading the light with R (red), G (green), B (blue) linear image sensors mounted on respective exist surfaces of the tricolor separating prisms. The subject is read in a main scanning direction by the linear image sensors while at the same time the subject is moved relatively to the linear image sensors in an auxiliary scanning direction that is substantially perpendicular to the main scanning direction, so that two-dimensional image information of the colors R, G, B can be produced.
The conventional color image reading systems with the tricolor seperating prisms are advantageous in the they can read one line on the subject simultaneously with the three linear image sensors. The conventional color image reading systems need such three linear image sensors, and the tricolor separating prisms are highly expensive. It requires a highly sophisticated technique to install the three linear image sensors on the tricolor separating prisms in optical alignment.
Therefore, the overall cost, including the installation cost, of the conventional color image reading systems with the three linear image sensors and the tricolor separating prisms is relatively large.
In an attempt to solve the above problems, there has recently been proposed a 3-line image sensor (or a 3-line linear image sensor) 1 of semiconductor package structure which, as shown in FIG. 9 of the accompanying drawings, R. G. B linear image sensors 1R, 1G, 1B fabricated parallel to each other on a single semiconductor substrate and housed in a single package. The R, G, B linear image sensors 1R, 1G, 1B has respective photodetectors comprising respective arrays of several thousand photoelectric transducer pixels joined in a main scanning direction X (also referred to as a "line direction"), and respective R, G, B filters (not shown) mounted respectively on the photodetectors.
The 3-line image sensor 1 is capable of, by itself, separating three colors from applied light and photoelectrically reading the applied light in the main scanning direction. The linear image sensor 1G and the linear image sensor 1B are spaced a distance Lbg of several tens .mu.m. for example, from each other in an auxiliary scanning direction Y perpendicular to the main scanning direction X, and the linear image sensor 1B and the linear image sensor 1R are spaced a distance Lrb of several tens .mu.m, for example, from each other in the auxiliary scanning direction Y. Usually, the distance Lbg is equal to the distance Lrb. Therefore, the linear image sensor 1G and the linear image sensor 1R are spaced a distance Lrg which is equal to 2.multidot.Lbg=2.multidot.Lrb.
With the 3-line image sensor 1 incorporated in a conventional color image reading system, the conventional color image reading system requires no tricolor separating prisms, and hence needs no time and process to install the image sensor on tricolor separating prisms.
However, since the linear image sensors 1R, 1G, 1B are spaced from each other in the auxiliary scanning direction Y. as shown in FIG. 10 of the accompanying drawings, they cannot simultaneously read an image X1 (extending in a direction normal to the sheet of FIG. 10) of a line on a subject 2 that is fed in the auxiliary scanning direction Y.
Specifically, light L containing image information on the subject 2 that is being fed in the auxiliary scanning direction Y is led through a focusing optical system 3 to the linear image sensors 1R, 1G, 1B of the 3-line image sensor 1. The image X1 which is borne by the light L is first read at a position YI along the auxiliary scanning direction Y by the linear image sensor 1R, and then read at a position Y2, spaced from the position Y1 by a distance Xrb, along the auxiliary scanning direction Y by the linear image sensor 1B. Finally, the image X1 is read at a position Y3, spaced from the position Y2 by a distance Xbg and from the position Y1 by a distance Xrg, along the auxiliary scanning direction Y by the linear image sensor 1G. In FIG. 10, the distances Xrb, Xbg, Xrg are related to each other such that Xrb=Xbg, Xrg=2.multidot.Xrb=2.multidot.Xbg. It should be noted that when the magnification (reading resolution) of the focusing optical system 3 is varied, the differences between the positions along the auxiliary scanning direction Y where the subject 2 is read are also varied.
The image information thus read is photoelectrically converted by the linear image sensors 1R, 1G, 1B into an image signal that is converted by an A/D converter (not shown) into digital image data. The digital image data are then stored in an external memory such as a hard disk or the like.
When the digital image data are stored in the external memory, it is preferable to store the digital image data in the external memory such that the differences between the reading positions of the linear image sensors 1R, 1G, 1B along the auxiliary scanning direction Y are compensated for, i.e., to store digital image data as files in the external memory.
There are two techniques available for storing digital image data in the external memory such that the differences between the reading positions of the linear image sensors 1R, 1G, 1B along the auxiliary scanning direction Y are compensated for.
For an easier understanding of these two techniques, it is assumed that the distance Xrb corresponds to 10.1 lines on the subject and distance Xrg corresponds to 20.2 lines on the subject.
According to the first technique, as shown in FIG. 11 of the accompanying drawings, the linear image sensor 1G which finally reads the image X1 is used as a reading position reference, and an output signal from the linear image sensor 1G is converted by an A/D converter 5G into digital image data Sg that is stored for each main scanning line directly into a memory 8 such as a hard disk or the like. The memory 8 is capable of storing digital image data of one frame.
The linear image sensor 1R which first reads the image X1 is connected through an A/D converter 5R to 21 line buffer memories 6R1.about.6R21 which comprise semiconductor memories for storing digital image data corresponding to 21 lines. The linear image sensor 1B which next reads the image X1 is connected through an A/D converter 5B to 11 line buffer memories 6B1.about.6B11 which comprise semiconductor memories for storing digital image data corresponding to 11 lines. Each of these line buffer memories is capable of storing data corresponding to more words than the number of the photoelectric transducer pixels, e.g., 5k words, each word being of bits, e.g., 16 bits, greater than the resolution of the A/D converter.
In order to store image data Sb from the linear image sensor 1B at the same reading position as the image data Sg of the linear image sensor 1G as the reading position reference, image data of the same pixel number of front and rear lines are read from the line buffer memories 6B10, 6B11 and then interpolated by an interpolator 7, and the interpolated image data that are 10.1 lines prior to the image data Sg of the linear image sensor 1G are stored as image data Sb in the memory 8.
Specifically, the interpolator 7 calculates image data Sb from the image data Sb10, Sb11 of the same pixel number of front and rear lines according to the following weighting interpolation equation (1): EQU Sb=0.1.multidot.Sb10+(1-0.1).times.Sb11 (1)
Similarly, the interpolator 7 calculates image data Sr from the image data Sr20, Sr21 of the same pixel number of front and rear lines according to the following weighting interpolation equation (2): EQU Sr=0.2.multidot.Sr20+(1-0.2).times.Sr21 (2)
According to the second technique, as shown in FIG. 12 of the accompanying drawings, no line buffers are employed, but image data Sr, Sg, Sb are stored in their entirety in a memory 8 such as a hard disk or the like, and then interpolated according to the above equations (1), (2) by a CPU 9. The interpolated image data are then stored as files at the same reading position in the memory 8.
Since the first technique shown in FIG. 11 uses the interpolator 7, it allows the image data Sr, Sg, Sb to be stored in real-time as files into the memory 8, and hence permits the stored image data to be processed at high speeds. However, since the many line buffer memories which comprise semiconductor memories are required, the first technique is highly costly to carry out. For high-resolution image data processing, because the speed at which the subject 2 is fed in the auxiliary scanning direction X is reduced, the differences, i.e., the distances Xbg, Xrg, between the reading positions on the subject 2, are increased, resulting in a need for more line buffer memories.
The second technique shown in FIG. 12 suffers a less memory cost because it needs no line buffer memories. However, the time required to carry out interpolating calculations according to the equations (1), (2) is considerably long, and hence the second technique fails to perform high-speed image data processing. As a result, it takes a long time until the image data are stored as files, resulting in a reduction in image data productivity.